The production of polymers utilizing renewable resources has been a field of increasing interest for many years. One particular area of interest concerns the production of polyesters that may be formed from polymerization of lactic acid-based monomers. Specifically, ring-opening polymerization of lactide has shown promise in production of polymeric materials. Lactic acid-based materials are often of particular interest as the raw materials can be derived from renewable resources (e.g., corn, plant starches, and canes).
Various approaches have been attempted to obtain lactide-based polymeric materials having desired product characteristics. For example, copolymerization with other materials and preparation of substituted polylactides have been examined extensively. For instance, Chen, et al. ('Synthesis and Characterization of [L]-Lactide-Ethylene Oxide Multiblock Copolymers', Macromolecules, 1997, 30(15), 4295-4301) have examined direct copolymerization of lactide with oxirane using a wide range of tin and aluminum based organometallic catalysts.
Unfortunately, the copolymers obtained by such methods and from such materials tend to have very limited applications. Limited applicability has been primarily due to the problematical processing characteristics as well as due to the ultimate product characteristics. For instance, materials obtained to date have low melt temperature (Tm) and higher brittleness than would be desirable in many applications.
Another problem encountered with lactic acid-based polymeric materials has been a lack of hydrolytic stability and difficulty in recycling the materials. For example, the hydrolytic stability of the polymers is often such that they degrade too quickly during use, rendering them unsuitable for many applications. In addition, lactide homopolymers are only aerobically recyclable, and thus difficult to remove from the waste stream.
What is needed in the art are ecologically friendly polymeric materials capable of displaying improved characteristics, including materials that may be produced from raw materials derived from renewable resources.